Which graphics card would permit eGPU rendering in Cycles with iMac?
Blender has stopped OpenCL (the API AMD uses) support for Mac because the compiler on macOS is very buggy. Apple also hasn't shown any effort to fix it, and instead favored their own API called Metal (which Blender does not support).
Blender does, however, support CUDA on MacOS meaning that if you have an Nvidia GPU that is CUDA capable and has a compute capability of 3.0+, you would be able to render with that card. However, recent macs do not use Nvidia cards anymore, so the only way would be to use an eGPU with an Nvidia card installed. You can also install Windows on your mac and just use the AMD card you have as window's OpenCL compiler has no big issues and is supported by Blender.